One class of opto-electrical devices is that using an organic material for light emission (an organic light emitting device or “OLED”) or as the active component of a photocell or photodetector (a “photovoltaic” device). The basic structure of these devices is a semiconducting organic layer sandwiched between a cathode for injecting or accepting negative charge carriers (electrons) and an anode for injecting or accepting positive charge carriers (holes) into the organic layer.
In an organic electroluminescent device, electrons and holes are injected into the semiconducting organic layer where they combine in to generate excitons that undergo radiative decay. In WO 90/13148 the organic light-emissive material is a polymer, namely poly(p-phenylenevinylene) (“PPV”). Other light emitting polymers known in the art include polyfluorenes and polyphenylenes. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). WO 99/21935 discloses the class of materials known as dendrimers. In a practical device one of the electrodes is transparent, to allow photons to escape the device.
A organic photovoltaic device has the same construction as an organic electroluminescent device, however charge is separated rather than combined as described in, for example, WO 96/16449.
One feature of OLED architecture that has attracted considerable research is the selection of the cathode. Factors to be taken into account in selecting a cathode include (a) the workfunction of the cathode relative to the lowest unoccupied molecular orbital (LUMO) of the emissive material and (b) the possibility of the cathode degrading the organic material or vice versa. It will therefore be apparent that selection of the appropriate cathode for a given material is not straightforward, and is yet further complicated when the cathode is required to be compatible with all three of a red, green and blue electroluminescent material as per a full color OLED. For example, Synthetic Metals 111-112 (2000), 125-128 discloses a full color display wherein the cathode is LiF/Ca/Al. The present inventors have found that this cathode is particularly efficacious with respect to the blue emissive material but which shows poor performance with respect to green and, especially, red emitters. For this cathode, the present inventors have found a particular problem of degradation in green and red efficiency when pixels of these colors are not driven which is believed to be due to migration of lithium into the electroluminescent material.
Some attention has been directed towards cathodes comprising barium. For example cathodes comprising elemental barium are disclosed in WO 98/57381, Appl. Phys. Lett 2002, 81(4), 634 and WO 02/84759, and cathodes comprising barium fluoride are disclosed in Appl. Phys. Lett. 2001, 79(5), 2001 and in the applicant's co-pending application PCT GB02/03882. However, barium containing cathodes still have drawbacks. In particular, the workfunction of barium is relatively high which makes injection of electrons into the LUMO of the typical blue electroluminescent material energetically unfavourable. Thus, barium is a relatively inefficient electron injector for blue electroluminescent materials at least.
It is therefore an object of the invention to provide an organic optical device wherein the cathode comprises a relatively high workfunction metal, but has improved performance, in particular improved lifetime (i.e. the time taken for the brightness of the device to decay to half its original brightness at a fixed current) and improved efficiency, as compared to prior art devices with cathodes comprising barium.